Soft-input soft-output decoding of variable length codes

We present a method for utilizing soft information in decoding of variable length codes (VLCs). When compared with traditional VLC decoding, which is performed using "hard" input bits and a state machine, the soft-input VLC decoding offers improved performance in terms of packet and symbol error rates. Soft-input VLC decoding is free from the risk, encountered in hard decision VLC decoders in noisy environments, of terminating the decoding in an unsynchronized state, and it offers the possibility to exploit a priori knowledge, if available, of the number of symbols contained in the packet     

An anti-noise modem for visible light communication systems using the improved M-ary position phase shift keying

In this paper, we propose an anti-noise modem based on the improved M-ary position phase shift keying (MPPSK) for visible light communication (VLC) systems. The line spectral interferences in the original MPPSK are theoretically analyzed and approximately mitigated by adopting a set of improved waveform samples. Then, within one symbol duration, the peak envelopes of the received signals are captured at each of synchronization bit with a quantity of sampling offsets to improve the proportion of the reliable data tones. In addition, a novel demodulation scheme using an amplitude-position detector based on soft decision is proposed to recover the MPPSK symbols, which can minimize the contamination influence of channel noise on symbol decision. With this scheme, the false alarm error and wrong slot error caused by the multipath fading in VLC channel can be effectively corrected and the bit error rate (BER) performance of the system is significantly improved. Simulation results demonstrate that the proposed scheme reduce the required signal-to-noise ratio (SNR) at least by 2 dB for BER = 10⁻⁴, which show the feasibility and validity of this anti-noise modem.     

Image transmission in an OFDM VLC system using symbol scrambling and chaotic Walsh-Hadamard precoding

Abstract- An image secure transmission scheme combining symbol scrambling and chaotic Wahsh-Hardmard precoding is proposed in orthogonal frequency division multiplexing (OFDM)-based visible light communication (VLC) system. In the proposed scheme, a two-dimensional (2D) logistic-sine-coupling map (LSCM) is employed to generate two chaotic sequences. One is used to scramble the 16 quadrature amplitude modulation (QAM) symbols, and the other is used to scramble the rows of standard Walsh-Hadamard transform (WHT). The simulation results show that the proposed scheme can not only reduce the peak-average-power ratio (PAPR) but also achieve the image secure transmission over OFDM-based VLC systems.     

A Low-Cost VLC Implementation for MPEG-4

In this brief, a low-cost technique to implement variable-length coding (VLC) is presented. Instead of using a large codeword table as with most traditional methods, we employ two smaller tables, symbol-address table and section-based table, to encode input symbols. Compared with previous methods, our VLC implementation can reduce about 17% to 38% of memory space for storing larger VLC tables in MPEG-4 application. Synthesis results show that the proposed VLC encoder and decoder occupy 19761 and 20187 gate counts, respectively.     

PAPR reduction for OFDM‐based visible light communication systems using proposed hybrid technique

Orthogonal Frequency Division Multiplexing (OFDM) is utilized with visible light communication (VLC) systems to decrease the impacts of inter‐symbol interference and to achieve communication with high speed of data transmission and huge bandwidth. In any case, the main problem in OFDM‐based VLC systems is high peak‐to‐average power ratios (PAPRs). This paper proposes a hybrid PAPR reduction technique based on signal transformation combined with clipping. The Hadamard transform is used in the proposed technique to reduce the PAPR without affecting the bit error rate (BER) of the VLC systems. The optimum clipping threshold at which the PAPR is reduced simultaneously with the improvement the BER of the VLC systems is also determined. The performance of the proposed system is assessed in terms of complementary cumulative distribution function and the BER. The obtained results demonstrate that the proposed procedure can simultaneously decrease the PAPR and achieve good BER performance compared to the OFDM‐based VLC system.     

Joint source-channel codes based on irregular turbo codes and variable length codes

Variable length codes (VLCs), used in data compression, are very sensitive to error propagation in the presence of noisy channels. To address this problem, several joint sourcechannel turbo techniques have been proposed in the literature. In this paper, we focus on pairs of source/VLC of low redundancy, i.e., when there is a good match between the source statistics and the length distribution of the VLC. It is a case not considered extensively in the literature so far and the classical concatenation of a VLC and a convolutional code is not satisfying. Through EXIT chart and interleaving gain analysis, we show that the introduction of a repetition code between the VLC and the convolutional code considerably improves global performance. In particular, excellent symbol error rates are obtained with reversible VLCs which are used in recent source codecs.     

Analysis of random multiple access transmission systems

For a very large number of small earth-stations with varying transmission time and light traffic, the random multiple access (RMA) transmission system is capable of providing simultaneous access to earth-stations by associating each station with a unique sequence of code symbols. Each information bit will be transformed into these code symbols, where each symbol is an entry in a discrete time-frequency matrix. In this paper we propose a system model for RMA transmission systems and illustrate the application of RMA code sequences. The analysis of this transmission system, which causes interference to the desired signal, is presented. The performance parameter considered is the signal-to-noise ratio (SNR) at the output of a correlation detector. The analysis shows that the SNR depends on the number of users, the energy per data bit and the design of the time-frequency matrix which determines the total numbers and values of the time-delay units associated with any carrier.     

Location-adaptive transmission for indoor visible light communication

A location-adaptive transmission scheme for indoor visible light communication (VLC) system is proposed in this paper. In this scheme, the symbol error rate (SER) of less than 10^-3 should be guaranteed. And the scheme is realized by the variable multilevel pulse-position modulation (MPPM), where the transmitters adaptively adjust the number of time slots n in the MPPM symbol according to the position of the receiver. The purpose of our scheme is to achieve the best data rate in the indoor different locations. The results show that the location-adaptive transmission scheme based on the variable MPPM is superior in the indoor VLC system.     

Symbol re-mapping method for improvement of TML-1 VLC coder

To improve the efficiency of variable length codeword (VLC) encoding in TML-1 for H.26L, the authors propose to adaptively change the mapping relationship between a symbol and its bit pattern in the VLC table, based on a local measurement of the symbol probability. Simulation results show that the proposed method gives a ~30 and 37% reduction in the average bit rate for macro block type and coded block pattern information, respectively     

Non-Uniform Time Slot Based Modulation for Visible Light Communication System

Recently, visible light communication (VLC) has got increasing attention in the field of indoor wireless communications. VLC has the obligation of lighting besides data communications. The IEEE 802.15.7 task group proposed a VLC modulation format named color shift keying in which data symbols are defined based on the diffused light wavelength. This paper presents a new modulation scheme for indoor wireless visible light communication system named non-uniform time slot modulation (NUTS). The NUTS modulation scheme defines the feeder signals of red, green and blue LEDs as rectangular pulses with appropriate width and their positions. In this method, the time axes of all the three light sources (channels) are slotted to some non-uniform durations that all symbols are spanned over these slots as orthogonal dimensions. The proposed scheme has some advantages with respect to other approaches such as low bit error rate and high bit rate. Furthermore, it provides appropriate synchronization capability. Besides, in the design of NUTS modulation scheme it has been tried to deliberate all practical constraints, as far as possible.     

An Efficient Variable-Length Code Construction for Iterative Source-Channel Decoding

We present a novel variable-length code (VLC) construction which exhibits an inherent error correcting capability due to the exclusive presence of codewords with even Hamming weight. Besides error robustness, the proposed code construction features a similar codeword length distribution as Golomb-Rice codes, and therefore, in particular for sources with exponentially distributed symbols, has good source compression properties at the same time. We show that in a source channel coding framework with outer source encoding, inner channel encoding with a recursive convolutional code, and iterative decoding the proposed VLC construction can lead to significant performance improvements compared to fixed-length source encoding with optimized mappings. In particular, simulation results for the AWGN channel verify that for Gauss-Markov sources a performance close to the theoretical limit can be achieved.     

Design and research of adaptive O-OFDM symbol decomposing with serial transmission system in visible light communication

Iterative signal clipping (ISC) and symbol decomposing with serial transmission (SDST) techniques decompose optical orthogonal frequency division multiplexing (O-OFDM) symbol into the fixed number of multiple small-amplitude symbols.However,at low O-OFDM symbol variances,the decomposition symbols contain a large amount of all-zero symbols without load information,which results in bit error rate degradation,optical power penalty and information rate reduction.Aim at this problem,an adaptive O-OFDM symbol decomposing with serial transmission (ASDST) system was proposed,the symbol decomposition was adaptively determined according to the O-OFDM symbol amplitude.Symbol decomposition was terminated until O-OFDM symbol was completely decomposed or the maximum symbol decomposition times were attained,thereby eliminated all-zero symbols and reduced the average symbol decomposition times.The performance of ASDST system was verified by theoretical analysis and Monte Carlo simulation.     

一种利用约束Viterbi算法改进IS95协议信道编码的方法

IS95具有两个速率集,在同一个速率集中,不同的速率其数据帧的长度不同,为了使进入正交调制的数据帧的长度相同,其采用了信号重复器.信号重复器进行简单的重复作用,使得带宽不能充分利用.文中提出的方法利用一个信号发生器,产生已知的信号进入卷积编码,卷积编码后的数据帧长度同样符合要求,而信号发生器产生的已知信号在译码端可以作为译码时的约束条件进行约束Viterbi译码.在BSC信道模型的仿真结果证明,其在较高误码率的情况下具有较好的性能.     

Joint source-channel turbo decoding of entropy-coded sources

We analyze the dependencies between the variables involved in the source and channel coding chain. This analysis is carried out in the framework of Bayesian networks, which provide both an intuitive representation for the global model of the coding chain and a way of deriving joint (soft) decoding algorithms. Three sources of dependencies are involved in the chain: (1) the source model, a Markov chain of symbols; (2) the source coder model, based on a variable length code (VLC), for example a Huffman code; and (3) the channel coder, based on a convolutional error correcting code. Joint decoding relying on the hidden Markov model (HMM) of the global coding chain is intractable, except in trivial cases. We advocate instead an iterative procedure inspired from serial turbo codes, in which the three models of the coding chain are used alternately. This idea of using separately each factor of a big product model inside an iterative procedure usually requires the presence of an interleaver between successive components. We show that only one interleaver is necessary here, placed between the source coder and the channel coder. The decoding scheme we propose can be viewed as a turbo algorithm using alternately the intersymbol correlation due to the Markov source and the redundancy introduced by the channel code. The intermediary element, the source coder model, is used as a translator of soft information from the bit clock to the symbol clock     

基于满意度移动模型的点军标显示自动避让方法

将点军标显示避让问题当作空间竞争问题求解,提出一种基于社会活动领域的满意度移动模型的点军标自动避让方法。该方法以点军标显示最小外接矩形重叠区域大小作为军标满意度,将点军标配置位置的四方向领域作为移动备选位置,通过多次迭代使得各点军标位于其满意度最大位置,将此满意度最大的点军标集合作为避让标绘的结果。实验结果表明,该方法能够较好解决点军标显示占位冲突,实现自动避让标绘。     

High accuracy frequency offset correction with adjustable acquisition range in OFDM systems

A new carrier frequency offset estimation scheme in orthogonal frequency-division multiplexing (OFDM) systems is proposed in this paper. Both the carrier frequency offset acquisition and tracking are based on a fixed-length training-symbol-block, which consists of multiple small identical training symbols. When each training symbol is shortened, the number of training symbols in the training-symbol-block should be increased accordingly to keep the total training-symbol-block length fixed. The proposed scheme extends Moose's estimator, where the estimation error is only dependent on total training symbol energy and cannot be reduced any more, once the total training symbol energy is determined. The proposed scheme can shorten each training symbol in a training-symbol block and select an appropriate estimator simultaneously, which can lead to further reduction of estimation error and increase of acquisition range, even with the total training-symbol-block energy being fixed. Performance analyzes for the proposed scheme in both the additive white Gaussian noise channel (AWGN) and the multipath channel are also presented in this paper. All estimators in the proposed scheme are conditionally unbiased, and simulation results demonstrate that they can work well both in the multipath channel and in the AWGN channel.     

A Novel Ternary CDMA Code for TPSK Modulation Scheme

In code division multiple access (CDMA), two or more chips are grouped together to form symbols and each symbol is transmitted during the symbol period. The phase shift keying (PSK) modulation techniques map the digital baseband data into two or more possible signals by varying the phase of a radio frequency (RF) carrier. The recently proposed PSK scheme called ternary PSK (TPSK) scheme can convey three possible symbols. In this paper, a novel ternary based CDMA sequence so-called large area synchronous even ternary (LAS-ET) sequence is introduced to increase spectrum efficiency in TPSK scheme. Its sequence duty ratio and cross-correlation are analyzed. The performance analysis of this sequence is compared with the large area synchronous (LAS) sequence in term of symbol error rate and chip error rate (CER) over various channel models. It is shown that TPSK scheme in LAS-ET sequence outperforms LAS sequence in terms of CER evaluation. At the same time, the spectrum efficiency is doubled when a pair of chips in LAS-ET sequence is mapped into one symbol.     

Optimal placement of known symbols for slowly varying frequency-selective channels

The problem of placing known symbols in a data stream for a slowly varying frequency-selective channel is considered from an information-theoretic perspective. Given the amount of redundancy associated with known symbols, placement schemes that minimize the outage probability are derived by assuming that the transmitted codewords consist of packets that are constrained to have the same known symbol placement. Under the assumption that each known symbol cluster is at least as large as /spl alpha/ /spl ges/ 2L + 1 (where L is the channel order), we show that the optimal placement is obtained by arranging the known symbols into as many clusters as possible and placing them such that the unknown symbol blocks are as equal as possible. It is shown that the optimal placement of known symbol clusters does not depend on the probability density of the channel. Numerical examples are used to illustrate the ideas and potential gains of using optimal known symbol placement.     

Advances and prospects in visible light communications

Visible light communication (VLC) is an emerging technology in optical wireless communication (OWC) that has attracted worldwide research in recent years. VLC can combine communication and illumination together, which could be applied in many application scenarios such as visible light communication local area networks (VLANs), indoor localization, and intelligent lighting. In recent years, pioneering and significant work have been made in the field of VLC. In this paper, an overview of the recent progress in VLC is presented. We also demonstrate our recent experiment results including bidirectional 100 Mbit/s VLAN or Li-Fi system based on OOK modulation without blue filter. The VLC systems that we proposed are good solutions for high-speed VLC application systems with low-cost and low-complexity. VLC technology shows a bright future due to its inherent advantages, shortage of RF spectra and ever increasing popularity of white LEDs.     

Pilot symbol assisted coding

Proposed is a novel technique, hereby referred to as pilot symbol assisted coding (PSAC), where a predetermined fraction of binary pilot symbols is interspersed with the channel-coded bits at the channel coding stage, instead of multiplexing the pilots with the data symbols at the modulation stage, as in classic pilot symbol assisted modulation (PSAM). It is demonstrated that the PSAC succeeds in gleaning more benefits from the pilot overhead investment, than just simply the capability of channel estimation such as in the PSAM technique.